The open-cycle magnetohydrodynamic (MHD) system of generating electrical power has been widely discussed as a possible means of improving fuel energy utilization. In the MHD system, a fuel such as coal is burned in a combustor to provide combustion gases at such high temperature (2500.degree.-2750.degree. C.) that a plasma is generated. The plasma is seeded with an electrically-conductive material, such as a potassium compound like potassium carbonate (K.sub.2 CO.sub.3) to increase electrical conductivity of the gases. This high-temperature, electrically-conductive plasma at pressures of approximately 4-9 atmospheres is accelerated then to a high linear velocity (0.7-0.9 Mach, for example) for passing through an open ended circumferentially walled MHD channel. Superconductive magnets outside of the channel direct high levels of magnetic flux crosswise through and along the channel. The electrically-conductive gases rapidly traverse the magnetic flux and thereby induce on the channel walls parallel to the flux a DC potential that is directly proportional to the conductivity and speed of the gases and to the square of the magnetic flux, and that is inversely proportional to the pressure of the gases. This DC power in turn is converted to AC power by an inverter or the like for normal transmission to end users.
The combustion gases discharged from the MHD channel will be at temperatures generally exceeding 2000.degree. C. and probably even as high as 2200.degree. C., and at velocities generally exceeding 0.5-0.8 Mach. A diffuser is used to convert the kinetic energy into thermal energy by recovering the pressure to atmospheric or slightly higher.
Known MHD systems then direct the hot combustion gases through high temperature radiant boilers that cool the gases somewhat by generating steam. The boiler is sized to allow the combustion gases to linger therein a short duration while remaining at a high temperature. By maintaining the combustion gases hotter than 1500.degree. C. for 1.5-2.0 seconds, the nitrogen impurities (NO.sub.X) can be decomposed to be within acceptable EPA limits. Some slag and other undesirable waste combustion components are separated out of the combustion gases as condensate on the boiler walls and can be discharged at the bottom of the boiler. The fuel-rich combustion gases from the radiant boiler are directed through a secondary combustor wherein additional air is added (stiochiometry is increased from 0.75 to 1.05 approximately) to complete the fuel combustion but at relatively low temperatures. The combustion gases are then directed through the bottoming cycle equipment that generates additional steam usable in conventional steam expansion devices, such as a steam turbine. The combustion gases then would typically be passed successively through an economizer and a low temperature air heater, wherein water and air respectively would be heated for later use somewhere in the system, while the combustion gases would be cooled to 150.degree.-200.degree. C. The cooled combustion gases then would be passed through the gas cleaners whereat residual seed, slag or other impurities would be separated out. Inasmuch as the seed (typically K.sub.2 CO.sub.3) is a very high cost item and is used in large quantities approaching even 15-25% of the quantity of coal burned, every effort is made to recover and reprocess the seed for subsequent reuse. The combustion gases are subsequently discharged via a conventional stack to the atmosphere.
To support coal combustion at the elevated temperature of 2500.degree.-2750.degree. C., the combustion must be with oxygen enrichment or with high temperature air. Oxygen enrichment would require additional components, viz., an oxygen generation facility, and energy inputs to operate the facility; thus both increasing the cost and reducing the overall efficiency significantly. It is possible to provide atmospheric air with sufficient temperature and/or pressure energy levels to produce the high combustion temperatures, but special air preheaters are needed. The existing technology provides at least two high-temperature air preheaters, each comprised of a chamber housing a matrix network of refractory material blocks, such as ceramic. The combustion gases are passed through one preheater to heat the blocks therein while atmospheric air pressurized by conventional compressors simultaneously is passed through the other preheater and takes heat from the already heated blocks therein. The pressurized air passing through these air preheaters is preferably at pressures of approximately four to nine atmospheres while the combustion gases are at approximately one atmosphere so that pressure confinement means has to be provided. Moreover, the gating mechanisms that alternately pass the combustion gases and the pressurized air, respectively, at large temperature and pressure differences, through the air preheaters are complicated and introduces specific and costly design problems. Frequent thermal cycling at these elevated temperatures moreover, is hard on the refractory material to the extent that the risk of shortened operating life is present. Of concern also is the fact that the high temperatures and generally fuel-rich conditions of the combustion gases greatly accelerate corrosion of the components, as well as slag and particulate build-up on the component walls.